O Hubspot ensina a calcular o ROI para um projeto de marketing, e como achei a idéia simples, acho que pode ser aplicada a outros tipos de projeto, portanto segue o trecho relevante:

ROI = ( Profit – Investment ) / InvestmentBut before you run off to Excel, lets make sure you’re taking all the relevant factors into account. When you make this calculation, are you really looking at profit, or are you focusing on revenue? This one is a bit tricky, so don’t worry – just make sure you’re using what is left over after sale has closed.Now lets give this a shot. You’ve just done a mailing campaign, and it cost you $500. It took you about an hour to put together, and you’d normally bill out at $100 for this (our Investment is now up to $600 total). It really doesn’t matter how many people the mailing went out to, but we’ve generated 50 leads from this effort, which our Sales team values at $20 per lead (this puts our Profit at $1000). So our ROI calculation looks like this:ROI = ( $1000 – $600 ) / $600 = 0.66Multiply by 100, and you’ve got a percentage, and for this effort, our return was 66.7%. On the surface, we made less than we spent. A good effort would be one where the result is greater than 1.0 – it means we made more than we spent. If your ROI is anything less than 100%, you’re probably better off putting your investment into an interest-bearing account of some kind.